The present invention relates to impedance pneumography which is a technique for monitoring the expansion and contraction of the thoracic cavity during respiration.
Typically, a low level, constant current carrier is passed through the thorax via two ECG electrodes, for example, the Right Arm and Left Arm ECG electrodes. As respiration of the patient occurs, the patient's baseline impedance will vary slightly causing an amplitude-modulated voltage to be developed which is then synchronously detected. In the prior art, the reference used for the synchronous detector is the same clock used to develop the constant current drive.
A synchronous demodulator detects both amplitude and also phase information. If the phase of the input signal differs from that of the reference signal by 90 degrees the filtered output will decrease to zero. In a constant current pneumography system, the voltage developed across the patient (the incoming signal) may not be in phase with the carrier current and its associated drive signal (the reference input). This phase differential depends on the phase angle of the load impedance (the patient) which in practice can vary arbitrarily. When the incoming carrier voltage from the patient is synchronously demodulated using the original carrier drive signal as a reference, this variable phase shift is detected along with the amplitude modulation creating undesirable noise in the output. In such a system the displayed respiration trace can be easily perturbed by moving the patient cable or even just walking next to the patient. This type of noise is undesirable in general and can upset the inspiration detector.